Current understanding on Villosiclava virens, a unique flower-infecting fungus causing rice false smut disease

Abstract

Villosiclava virens (Vv) is an ascomycete fungal pathogen that causes false smut disease in rice. Recent reports have revealed some interesting aspects of the enigmatic pathogen to address the question of why it specifically infects rice flowers and converts a grain into a false smut ball. Comparative and functional genomics have suggested specific adaptation of Vv in the colonization of rice flowers. Anatomical studies have disclosed that Vv specifically infects rice stamen filaments before heading and intercepts seed formation. In addition, Vv can occupy the whole inner space of a spikelet embracing all floral organs and activate the rice grain-filling network, presumably for nutrient acquisition to support the development of the false smut ball. This profile provides a general overview of the rice false smut pathogen, and summarizes advances in the Vv life cycle, genomics and genetics, and the molecular Vv-rice interaction. Current understandings of the Vv-rice pathosystem indicate that it is a unique and interesting system which can enrich the study of plant-pathogen interactions. Taxonomy: Ustilaginoidea virens is the anamorph form of the pathogen (Kingdom Fungi; Phylum Ascomycota; Class Ascomycetes; Subclass Incertae sedis; Order Incertae sedis; Family Incertae sedis; Genus Ustilaginoidea). The teleomorph form is Villosiclava virens (Kingdom Fungi; Phylum Ascomycota; Class Ascomycetes; Subclass Sordariomycetes; Order Hypocreales; Family Clavicipitaceae; Genus Villosiclava). Disease symptoms: The only visible symptom is the replacement of rice grains by ball-shaped fungal mycelia, namely false smut balls. When maturing, the false smut ball is covered with powdery chlamydospores, and the colour changes to yellowish, yellowish orange, green, olive green and, finally, to greenish black. Sclerotia are often formed on the false smut balls in autumn. Identification and detection: Vv conidia are round to elliptical, measuring 3-5 μm in diameter. Chlamydospores are ornamented with prominent irregularly curved spines, which are 200-500 nm in length. The sclerotia are black, horseshoe-shaped and irregular oblong or flat, ranging from 2 to 20 mm. Nested polymerase chain reaction (PCR) and quantitative PCR have been developed to specifically detect Vv presence in rice tissues and other biotic and abiotic samples in fields. Host range: Rice is the primary host for Vv. Natural infection by Vv has been found on several paddy field weeds, including Digitaria marginata, Panicum trypheron, Echinochloa crusgalli and Imperata cylindrica. However, the occurrence of infection in these potential alternative hosts is very rare. Life cycle: Vv infects rice spikelets at the late rice booting stage, and produces false smut balls covered with dark-green chlamydospores. Occasionally, sclerotia form on the surface of false smut balls in late autumn when the temperature fluctuates greatly between day and night. Both chlamydospores and sclerotia may serve as primary infection sources. Rainfall at the rice booting stage is a major environmental factor resulting in epidemics of rice false smut disease. Disease control: The use of fungicides is the major approach for the control of Vv. Several fungicides, such as cuproxat SC, copper oxychloride, tebuconazole, propiconazole, difenoconazole and validamycin, are often applied. However, the employment of resistant rice cultivars and genes has been limited, because of the poor understanding of rice resistance to Vv. Useful websites: Villosiclava virens genome sequence: http://www.ncbi.nlm.nih.gov/Traces/wgs/?val=JHTR01#contigs.

Keywords: Ustilaginoidea virens; biotroph; effector; epiphytic growth; flower-infecting; grain-filling; rice false smut ball.

Figure 1

Typical symptoms of rice false smut disease caused by Villosiclava virens (Vv). False smut balls are formed in Vv‐infected rice panicles. During disease progression, the colour of false smut balls changes from white (a) to yellowish (b) and, ultimately, to greenish black (c). Sclerotia (inset) are usually formed on false smut balls in autumn. Arrows and arrowheads indicate false smut balls and sclerotia, respectively.

Figure 2

Villosiclava virens (Vv) undergoes blastogenesis and epiphytic growth and can form false smut balls in a rice male‐sterile mutant. (a–d) Representative epifluorescent microscopy images showing epiphytic growth and/or blastogenesis of a green fluorescent protein (GFP)‐labelled Vv isolate on detached leaves of rice (a, Oryza sativa accession Pujiang 6), paddy field weed Echinochloa crusgalli (b) and abiotic surface of cellophane (c, d). White arrows indicate Vv hyphae. c, conidium; sc, secondary conidium. Bars, 50 μm (a, b), 10 μm (c, d). (e) A representative image showing false smut balls on the rice male‐sterile mutant abcg15 (Qin et al., 2013). The Vv isolate P4 was inoculated at the late booting stage and the photograph was taken at 17 days post‐inoculation. White arrows indicate the white sterile anthers in abcg15 flowers. fsb, false smut ball.

Figure 3

A schematic diagram showing how Villosiclava virens (Vv) converts a rice spikelet to a false smut ball. Vv spores landing on the top of the second leaf (l) can enter the developing panicle (p) along with rainwater (①). Subsequently, Vv spores are attached to the developing spikelet (sp) and germinate to produce hyphae (②). The hyphae extend into the inner space of the spikelet via the gap between the palea (pa) and lemma (le) (③). Primary infection sites are found in the stamen filaments (sf) (④), where the hyphae extend intercellularly in both directions (⑤). Hyphae can extend into anthers (an) and reach the rachilla (ra), and intertwine with the ovary (o) and other floral organs. Vv hyphae acquire abundant nutrients to form false smut balls (fsb), hypothetically through hijacking of the rice nutrient reservoir (⑥). This step (arrow with dotted line) is of particular interest and requires further exploration. Red curved lines represent Vv hyphae. Red arrowheads point to the primary infection sites at the stamen filaments.